Interpretive Summary: In the Great Plains of North America, cattle grazing is one of the primary economic uses of rangelands. Understanding how grazing effects critical ecosystem processes is important for long range planning and management of rangeland ecosystems. In this study, we measured the important ecosystem properties of plant community photosynthesis, soil respiration, production of green photosynthetic tissue, and plant species composition of longterm (56 yrs) grazed and ungrazed pastures on the shortgrass steppe of northeas- ern Colorado. Three years of growing season data showed that longterm graz- ing caused a shift in plant species composition to a greater percentage of warm season grasses than in the ungrazed plots. There was also a slight re- duction in the total number of species in the grazed plots. In the heavily- grazed plots this change in species composition resulted in early season community photosynthesis rates which varied with precipitation and tempera- -ture. However, over the three years there was very little effect of grazin on photosynthesis in heavily or lightly grazed pastures. There was also no difference in soil respiration or production of green photosynthetic tissue in the grazed vs. ungrazed pastures. Overall, this study suggests that 56 years of cattle grazing has not altered plant and soil processes which con- trol carbon exchange of the shortgrass prairie. This ecosystem seems to be quite tolerant of grazing, probably because it was historically grazed by bison and other large animals. Therefore, the native plant species have developed a tolerance to grazing. This research supports the notion that properly managed cattle grazing is a sustainable economic usage of the shortgrass prairie of North America.

Technical Abstract:
In the Great Plains of North America, cattle grazing is one of the primary economic uses of rangelands. In order to better understand how grazing aff- ects the functioning & sustainability of the shortgrass steppe, we measured C02 exchange rates (CER) on pastures with a 56-yr history of heavy & light grazing (70% & 25% removal) & their accompanying 0.8 ha exclosures, in NE Colorado. A CER chamber that covered one M2 of native grassland was used o 5 plots in 4 pastures in 1995,96 & 97. Soil respiration was also measured with a green vegetation index (GVI) & plant species composition. Longterm cattle grazing caused a shift in plant species composition. Exclosures had more cool-season (C3) species than grazed plots, which contained more warm- season (C4) grasses. There were few differences in GVI of grazing and excl- osed pastures, due to the dominance of blue grama (Bouteloua gracilis, C4) in grazed pastures. Seasonal differences in CER between the grazed pastures s& exclosures were not well related to GVI, but were driven by climate vari ability & species composition differences induced by longterm grazing. The unusual cool, wet spring of 1995 was favorable to cool-season plant metabo- lism & resulted in higher CER in the exclosures compared with grazed past- ures. Warm, dry conditions early in 1996 favored warm-season species, resu- lting in higher CER in the heavily-grazed pasture. In 1997, climate & spec- ies composition combined such that there was little difference in CER bet- ween the grazed & exclosed pastures. Soil respiration was similar in grazed & exclosed plots in all 3 years. This study suggests a longterm history of grazing has altered plant & soil processes which control carbon exchange of the shortgrass prairie. A history of grazing by large herbivores on the shortgrass steppe appears to have conferred a resilience to cattle as well.